The invention relates to a colour display tube comprising an electron gun of the "in-line" type for generating three electron beams whose axes are coplanar, a deflection unit comprising a first and a second deflection rail for generating a first and a second deflection field for deflecting the electron beams in the operating display tube across said display screen in two mutually perpendicular directions, the direction of the first deflection field being parallel to said plane, and convergence correction means which, during deflection, keep the electron beams converged on a display screen arranged on a wall of the envelope and having a short display screen axis and a long display screen axis.
Various types of deflection units may be used for deflecting the electron beams in colour display tubes. In tubes having an "in-line" electron gun system these deflection units are generally self-convergent. In conventional deflection units having line deflection coils of the saddle type and field deflection coils of the toroidal type the winding technique used does not permit of rendering the unit entirely self-convergent. Usually such a winding distribution is chosen that a certain convergence error remains, which error is commonly referred to as coma. This coma error becomes manifest, for example, in a larger raster (horizontal and vertical) for the outer beams with respect to the central beam. The horizontal and vertical deflection of the central beam is smaller than that of the outer beams. As described, inter alia, in U.S. Pat. No. 4,196,370, this can be corrected by providing elements (referred to as field shapers) of a material (for example of mu-metal) having a high magnetic permeability around the outer beams at the end of the gun. The peripheral field at the area of the outer electron beams is shielded to some extent by these elements, so that these beams are deflected to a lesser extent and the coma error is reduced.
Various problems then present themselves. A first problem is that the shielding means around the outer electron beams detrimentally influence the spot shape of these beams. Another problem is that the correction of the field coma (Y coma) is anisotropic. In other words, the correction in the corners is less than the correction at the end of the field axis (the field axis is the display screen axis which is parallel to the field deflection direction). This is caused by the positive "lens" action of the line deflection coil (approximately quadratically with the line deflection) for vertical beam displacements. (The field deflection coil has a corresponding lens action, but it does not contribute to the relevant anisotropic effect.)